I took the car back to Mark at Benztek in Tucson. All the R134 had leaked out and some oil was leaking out too. Mark found some seals (I think, sorry, I'm no expert) near the compressor, like above it maybe, that were leaking. He replaced those and refilled the R134. He didn't charge me since he is not sure that there is not a leak at the compressor (which was not changed during the R12 -> R134 conversion). I'll drive it for a few days and have a look under the compressor, it should be obvious if oil is leaking as it's all cleaned up now.